Release of iodine in the atmospheric oxidation of alkyl iodides and the fates of iodinated alkoxy radicals

This paper describes a study of the products of the Cl-atom-initiated oxida tion of three alkyl iodides, RI = CH3I, C2H5I, and 2-C3H7I, carried out in synthetic air at atmospheric pressure and at room tempera;un. Fourier-trans form infrared spectroscopy was used to follow the decay of reactants and su bsequent formation of products. The primary step proceeds via two channels, one of which yields HCl and an iodinated alkyl radical, and the other I at oms and an alkyl chloride. Quantitative analysis of the product yields, tog ether with an assessment of the formation of HCl in secondary processes, al lowed the fractional branching into the two channels to be calculated. The channel yielding HCl from RI constitutes a fraction 0.59, 0.93, and 0.68 fo r R = (CH3, C2H5, and 2-C3H7. The iodinated alkyl radical forms first a per oxy, and then an alkoxy, radical in the presence of air. The final products CH2O, CH3CHO, and CH3COCH3 were observed as expected for the decomposition of these radicals with RI = CH3I, C2H5I, and 2-C3H7I, and the fractions of the alkoxy radicals fragmenting to the carbonyl compounds were 0.88, 0.57, and 0.86, respectively. Atomic iodine is formed concomitantly with the car bonyl species, so that these fractions also indicate the yield of I atoms i n the secondary process. Alternative reaction pathways for the iodinated al koxy radicals, in particular reaction with O-2, are evaluated and discussed . The yields of I atoms in the primary and secondary steps, taken in combin ation with kinetic data, make it possible to estimate the contribution of t he Cl-initiated oxidation of the alkyl halides to I-atom production in the atmosphere land, making certain assumptions, the analogous contribution fro m OH-initiated oxidation). Radical-initiated processes might augment the ph otolytic yield of I atoms from simple alkyl iodides: the maximum enhancemen ts lie between 5% (CH3I) and more than 30% (2-C3H7I). (C) 2001 Elsevier Sci ence Ltd. All rights reserved.